This tale, perhaps apocryphal, is one that CIA officers still tell with glee:

It's the mid-1950s, and two elderly lawmakers sit in a Senate subcommittee hearing room, listening to the CIA's deputy director of operations drone through his prepared statement on the agency's latest covert action. One of the senators rests his head in his arms and falls asleep. His colleague stares blankly at the briefer, stealing glances at a newspaper.

The CIA official raises his voice for a moment, more to relieve his own boredom than to stir his audience. "Paramilitary activities," he intones, "have been an important part of our program since the early days of the Cold War."

The slumbering senator awakens with a start. "Parliamentary activities?" he bellows. "You fellas can't go messin' round with parliaments. I won't have it!" With that, the octogenarian rose and left the room.

The moral of the story is evident: Not only did the CIA's legislative watchdog have no teeth, it was sound asleep.

Attorney General Eric Holder's appointment last week of a special prosecutor to investigate CIA prisoner abuses is but the latest of many efforts to rein in the agency. I've been a witness to some of those efforts, as an assistant to Sen. Frank Church of Idaho during his committee's investigations in the 1970s and later as an aide to former defense secretary Les Aspin when he led a probe of agency structures in the aftermath of the Aldrich Ames spying scandal.

Such inquiries can prove useful, leading to critical reforms, stronger oversight and, perhaps most important, changed attitudes about the CIA and other intelligence agencies. But I've also learned that high-profile investigations will not transform human nature, turning intelligence officials -- or the presidents and White House aides who direct them -- into angels, unsusceptible to zeal and folly. Even when the watchdogs on Capitol Hill or in the Justice Department awaken, intelligence abuses and scandals will recur. We will launch new investigations and introduce new reforms, but sometimes all we can do is clean up the messes after the fact. So let's get used to it.

During the first half of the Cold War, the CIA was largely free of serious congressional supervision. And despite controversies such as the U2 shoot-down over the Soviet Union, the Bay of Pigs fiasco and the covert funding of the National Student Association, the agency escaped widespread public opprobrium. That changed in 1974, when the New York Times's Seymour Hersh reported that the CIA had deployed its dark arts at home, spying on American citizens. In response, Senate leaders tapped Church to lead the first major investigation of the CIA.

The Church Committee discovered that intelligence abuses ran far deeper than initially reported. The CIA had indeed spied on Vietnam War dissenters at home, but the FBI had gone further, disrupting the lives of antiwar protesters and civil rights activists. It was "a road map to the destruction of American democracy," committee member Walter Mondale said during a public hearing.

Church was equally appalled by the overseas excesses of the CIA, including covert actions against democratic regimes -- such as Chile's -- and assassination plots. He blasted the agency for "the fantasy that it lay within our power to control other countries through the covert manipulation of their affairs."

At the time, Church told me that when he first came to the Senate in 1957, "some of those senior senators who did have this so-called [CIA] watchdog committee were known to say, in effect: 'We don't watch the dog. We don't know what's going on, and furthermore, we don't want to know.' " He vowed to take a new approach but found it difficult to learn exactly who told the CIA to do what -- a dilemma the nation faces today in trying to understand who ordered secret prisons, harsh interrogations and extraordinary renditions.

After one of our closed sessions on CIA assassination plots, Church leaned back in his chair and rubbed his forehead in frustration. Who had ordered the plots against Patrice Lumumba of Congo and Fidel Castro of Cuba? The president? The director of central intelligence? Someone lower down in the CIA? The testimony from the agency's witnesses was filled with ambiguities. " 'Could,' 'would,' 'probably,' 'assume,' 'might,' 'have a feeling,' " Church fumed to a particularly evasive (or forgetful) witness. "And we're talking about a matter of such grave importance as assassination!" The committee was never able to pinpoint responsibility.